The present invention has as an aim of providing a device for scanning register marks into a polychrome printing machine processing a sheet or a web material. This material, or print substrate, usually has an area for printing the image and a printing area for the accuracy control marks, marks usually known under the name of register marks, related to the setting into register of the different printing colors.
Such machines comprise indeed several printing units the ones following the others, each one printing on the substrate, by means of an engraved cylinder or a plate cylinder for example, a same pattern of a different color. For obtaining a perfect final image, it is necessary for all the prints of different colors to be exactly superimposed. The register control of these prints is achieved by means of register marks printed by each printing cylinder within the area intended for quality control marks, thus usually in the margin of the worked substrate. Thanks to a scanning device, these marks allow to determine the misregister of each color compared to the color of the first printing unit, usually used as reference. To compensate these shifts, a correcting order is issued and works either on the path of the printing substrate, or on the location of the corresponding printing cylinders.
Many known devices, such as those described in documents CH690096, EP0401691 and U.S. Pat. No. 5,747,795, allow to register and scan these marks printed on sheet or web elements traveling in front of a light source. However these devices can usually scan only one register mark at the same time, which means that a polychrome print i.e. requires as many scanning devices as there are marks, that is to say colors into the print.
Several devices, such as the one described in the document EP0214214, allow to take a picture of a whole range of marks by means of a video camera like a CCD one, then to operate on this image an analog-to-digital conversion, to center this digitized image on a scanning gate and to determine variations compared to reference marks. A white light source ensures a sufficient lighting of the substrate filmed by the video camera. This light source can result from a stroboscope which, thanks to its repeated flashes, allows to take fixed images of the substrate travelling at high speed.
Other devices, such as the one described in document EP0512448, propose to solve problems of selecting register marks which have the characteristic to be slightly contrasted with regard to the background color of the substrate on which they are printed; usually when the printed colors fade to paleness such as it is the case for example with pastel yellow, cream or light blue. The above mentioned device allows to scan only one mark at a time, the latter being illuminated by a white colored light source. The light reflected by this mark is separated by two channels made of optical fibers at the end of which two filters of different colors are arranged and located in front of two photosensitive units. Each photosensitive unit is especially sensitive within a frequency range of a distinct color and produces an electric signal at the time of the register mark travelling. The mark scanning is achieved by means of a comparting/selecting device which selects, among the generated electric pulses, the more representative one for the color mark.
When the aim is the simultaneous scanning of several register marks by means of the same device, the lighting of these marks becomes an increasingly significant component, particularly when a single, white or monochromic light source cannot make these marks more visible. Indeed, according to the color of the printed marks, the latter seem likely, under such a lighting, not to be sufficiently contrasted and to appear as invisible or, on the contrary, to generate dazzling or reflecting problems in the presence of specular colors such as gold color marks for example.
In the case used colors are intense and clearly allow to distinguish the printed marks by well shaped contours, the simultaneous scanning, by a same device, of several marks equipped with such colors would not cause in fact a particular problem; the latter being easily recognizable under a single white light as shown for the device of document CH686501.
Hence, in a whole third of cases, the printed colors are not so distinguishable from each other and require specific lightings in order to improve the real contrast either between themselves or in accordance with the background color of the printed pattern. Thus, a mark with a prevalence of green, purple or orange will appear all the more contrasted when its lighting color is full of complementary color, that is to say respectively in red, yellow or blue for the case.
In order to guarantee the reliability and the performance of the scanning systems, it is also obvious to make these distinctive marks quite apparent. Indeed, at the time of the start up of the printing machine, the first stage comprises the searching of the initially unknown positions for each register mark. This process is easier when each of the marks is illuminated by a source of appropriate color. In the same way, when these marks travel at significant speeds, i.e. up to 20 m/s, one will easily note that it is also obvious, even necessary, that these marks can be scanned without any possible doubt.
Currently, the simultaneous scanning of two or three register marks of slightly contrasted colors must be carried out by as many scanning devices; each one being equipped with a specific lighting according to the color mark for which it is intended. However, such a plurality of devices increases the printing machine installation and maintenance costs, requires more space and includes a scanning system more difficult to deal with in its whole embodiment, while proportionally increasing the risk of possible breakdowns.